1. Field of the Invention
The present invention relates to ladders and support members for ladders and, more particularly, relates to a ladder assembly including a single rung ladder suitable for allowing access into a chamber with a confined area entry adjacent a support surface.
2. Description of the Prior Art
Subterranean chambers, such as manholes, are sometimes equipped with rungs permanently secured to the side walls of the chamber for facilitating personnel access into the chamber. The ladder effectively formed by such rungs is, however, expensive to manufacture and maintain, since the ladder is infrequently used during the life of the manhole. Also, such permanent rungs create a safety concern since the rungs can easily rust to the point of structural failure, and can become coated with dirt, grease, and other debris. Moreover, entry into the manhole is difficult and can be unsafe, since the person may not be properly supported until he descends far enough for his hands to grasp the top rung. Finally, rungs permanently positioned in a manhole are difficult to inspect, and are costly to repair.
Various types of portable ladders have been devised for allowing human access into chambers below a support surface for the ladder. Portable ladders are thus often used to allow repair personnel to enter a manhole to gain access to subterranean equipment, even in situations where the manhole was previously supplied with rungs permanently secured to the sidewalls of the manhole. Such portable ladders enable safer and easier access to the manhole, in that the ladder may extend upwardly from the ground level while in use. Moreover, the ladder assembly is portable and thus may be removed after the repair task is completed, and reused at another manhole. Between uses, the portable ladder assembly may be easily checked and, if necessary, cleaned and/or repaired. Thus those familiar with the cost of providing human access into subterranean chambers, such as manholes, recognize the low cost and safety benefits associated with portable ladders.
A significant problem with prior art portable ladders used for obtaining access to manholes relates to the confined area entry of the manhole. The opening to many manholes is only 22 inches in diameter, and the addition of a standard dual rail ladder into this restricted area makes human access, at best, difficult. The extension ladder disclosed in U.S. Pat. No. 1,248,189 is thus complicated and costly, and will practically prohibit an individual from entering a standard manhole.
U.S. Pat. No. 3,120,290 discloses a ladder assembly intended for manhole entry. The ladder contains standard spaced rails with each rung connected at one end to one of the rails. Accordingly, the ladder restricts the entry area into the manhole so that access is difficult or practically impossible. Moreover, apertures provided in the sides of each of the rails for facilitating depth adjustment of the ladder within the manhole substantially weaken the ladder, and are thus not permitted according to various governmental and/or industrial standards.
Another problem with portable ladder assemblies intended for enabling manhole entry relates to the necessary structural relationship between the manhole and the ladder assembly. The portable ladder disclosed in U.S. Pat. No. 2,965,195, for example, uses cutouts in a manhole riser to secure the ladder in place. Such cutouts not only increase the cost of a conventional manhole riser, but allow water to flow into the manhole. U.S. Pat. No. 4,146,144 discloses a manhole ladder assembly with dual rails which structurally cooperates with the manhole riser. Manholes often vary in design and size, however, and the ladder assembly can thus only be safely used with a manhole riser design similar to that shown in the patent. Accordingly, many existing manhole risers would have to be modified so that they could cooperate with the ladder assembly to serve its intended purpose. Similarly, a portable ladder entry system depicted in German references G8319615.3 and G8319616.1 apparently require that special components be affixed to the manhole riser and/or that the manhole riser cooperate with such components to enable the ladder assembly to be used.
A single rail ladder is disclosed in U.S. Pat. No. 4,069,892, which is suitable for use in wells and other vertical shafts. The ladder is, however, intended as a permanent installation in the well, and each end of the ladder includes a lug for fastening the ladder to sidewalls of the well. A single rail ladder referred to as a pompier ladder includes an upper hook with teeth. Although this ladder has the advantage of not substantially restricting the entry into a manhole, the ladder still requires that the upper hook be able to cooperate with the manhole riser to hold the ladder in place. Moreover, this type of ladder does not extend vertically substantially above the riser, and thus entry by personnel into the manhole is difficult.
The disadvantages of the prior art are overcome by the present invention, and an improved portable ladder apparatus is hereinafter disclosed which may be used for enabling access into a restricted area manhole without regard to the size, shape, design, or material of the manhole. The ladder meets various governmental and industry standards, and may be safely used with other rescue and material handling equipment intended to allow ingress or egress of personnel or equipment.